System and method for improved steam turbine operation

ABSTRACT

An electric power plant steam turbine system with programmed digital computer control in which excessive rotor stress and strain are prevented by developing control signals based upon a comparison between present calculated heat flow and a reference heat flow, the turbine operation being controlled by the control signals so that strain is maintained substantially at the maximum allowable value.

United States Patent [19] [11 3,928,972

Osborne Dec. 30, 1975 [54] SYSTEM AND METHOD FOR IMPROVED 3,359,732 12/1967 Schuetzenduebel 60/105 AM TURBINE OPERATION 3,561,216 2/1971 Moore, Jr. 60/105 X 3,577,733 5/1971 Manvel 60/105 [75] Inventor: Robert L. Osborne, Wallingford, Pa. [73] Assignee: Westinghouse Electric Corporation, Primary ExaminerMartin Schwadron Pittsburgh, P Assistant Examiner-Allen M.. Ostrager Attorney, Agent, or Firm-E. 1F. Possessky [22] Filed: Feb. 13, 1973 [21] Appl. No.: 331,738 [57] ABSTRACT An electric power plant steam turbine system with 52 US. Cl 60/646; 60/660 programmed digital Computer control in which exces- 51 Int. Cl F01k 13/02 Sive rotor stress and Strain are Prevented y p- [58] Field of Search 60/646, 657, 660 g control Signals based "P a Comparison between I present calculated heat flow and a reference heat [56] References Ci d flow, the turbine operation being controlled by the UNITED STATES PATENTS control signals so that strain is maintained substantially at the maximum allowable value. 3,338,053 8/1967 Gorzegno 60/105' 3,358,450 12/1967 Schroedter 60/105 50 Claims, 6 Drawing Figures as T THROTTLE x 54 18 ERFiSSURE IMPULSE E ECTOR CHAMBER STEAM g??? 40 TEMPERATURE c OR DETECTOR as assets DETECTOR DETECTORS INLET STEAM VALVES GENERATING GENERATOR SYS TEM sEcTIoN 3 HYDRAULIC INLET VALVE REHEAT ACTUATORS VALVES l; L HYDRAULIC REHEAT VALVE AcTuAToRs T SL/til I H CONTROLS ED PSQ JTIQJ N TOR CONTROLS 4e HIGH PRESSURE HYDRAULIC LUID SUPPLY SYSTEM AND METHOD FOR IMPROVED STEAM TURBINE OPERATION CROSS-REFERENCE TO RELATED APPLICATION System and Method for Operating a Steam Turbine and an Electric Power Generating Plant, by T. C. Giras and M. E. Birnbaum, Ser. No. 722,779, filed Apr. 19, 1968, now abandoned, and continued as Ser. No. 124,993 on Mar. 16, 1971, and assigned to the present assignee.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to elastic fluid turbine systems and more particularly to systems and methods for controlling the dynamic operation of steam turbines as a function of heat flow.

2. Description of the Prior Art This invention constitutes an improved system and method for steam turbine operation, and as such pro vides a modification of, and improvement over, the system described in US Pat. No. 3,558,265, System and Method for Providing Steam Turbine Operation with Improved Dynamics, by William R. Berry, and assigned to the present assignee. This reference patent, hereinafter referred to as the Berry patent, provides a thorough disclosure of the background of the steam turbine art, and particularly of the effects of thermal loading on permissable turbine operation, and is incorporated by reference for the purposes of indicating the background of this invention and illustrating the state of the art upon which this invention improves. Specific reference is made at this point to the section of the Berry patent titled Background of the Invention," Col. 1, line to Col. 3, line I7, as well as the discussion of rotor thermal stress and plastic strain analysis' and its connection with turbine dynamics which appears from Col. 9, line 36 through Col. 12, line 49.

The Berry patent discloses an improved method of determining present rotor stress as a function of monitored tu'rbine impulse chamber steam temperature, comparing the present stress with a predetermined stress limit, and deriving a control signal from such comparison, by which inlet steam flow is controlled. In the system described, limits of impulse chamber steam temperature maybe further controlled by considerations of bore loading or casing strain. The effects of thermal expansion and contraction on respective regions of the turbine are thus controlled as a function of calculated present stress at such regions, which calculations are based upon the monitored inlet steam condition, centrifugal force loadings, and other input variables.

Another specific prior art example of programmed turbine control based upon considerations of present condition is compared with a limit which may be a predetermined limit or a future predicted stress limit calculated on the basis of the turbines temperature history. Such prior art techniques provide feedback control directed to dynamic loading and/or speed changing without exceeding allowable stress conditions. They are premised on present calculations of stress and not on any variable which is determinative of future stress. Such prior art systems place limits on the turbine operation without commanding that the desired changes be accomplished by the shortest possible sequence within such limits. For example, when turbine speed or acceleration is varied due to a control signal derived from a present calculation of rotor stress, the change in rotor stress due to the operating change is necessarily delayed due to thermal energy storage in the rotor metal. The lag between changes in temperature, at any given region of the turbine which is being monitored, and the turbine operation which is being controlled in response to present stress calculations, necessarily results in less than optimum control, even though the feedback system is fast and accurate. This lagging effect has been observed to lead to some oscillation tendency, and thus variations of thermally induced strain in certain turbine regions.

It is well understood that, no matter how efficient and responsive are the building blocks of a control system, the overall responsiveness is limited by the manner in which the information content of the input variables is utilized. There remains a clear need for a control system having the capacity to determine, from temperature measurements in respective regions of the turbine (such as the rotor), not only present turbine stress in relation to stress limits, but a more primary condition which relates to and is determinative of future changes in stress, and for a system having the capacity to control such primary condition so that quicker and more accurate turbine operation can be obtained while minimizing the oscillations and errors inherent in the prior art systems.

SUMMARY OF THE INVENTION In accordance with the broad principles of this invention, there is provided means for determining the temperature of turbine steam in at least one predetermined steam flow region associated in direct heat transfer relation with a predetermined turbine rotor portion. Improved control of the turbine is provided by combining the steam temperature determining means with means for determining a real time representation of heat flow to the rotor portion. The calculated heat flow representation is compared with a reference heat flow representation, which reference is selected as corresponding to maximum allowable rotor strain, and a comparison signal thus derived is processed by control signal means to provide a control signal for maintaining the turbine speed or load in such a way as to optimize the heat flow to the rotor at the value which maintains rotor strain substantially at the maximum allowable limit. Since heat flow to the rotor is a direct determinant of future rotor temperature, and consequently rotor strain, the system of this invention provides an advantage over previously used systems by immediately modifying the turbine operation to maintain rotor strain substantially equal to the maximum allowable value, as contrasted to present stress control wherein a differential between present rotor strain and the maximum allowable value of rotor strain is the basis for 3 limiting turbine operation.

In another form of this invention, the steam temperature is determined in one or more predetermined re gions, the heat flow to the rotor surface and present rotor surface strain are determined, and a limit is placed on the rate at which the turbine operating level is changed in order to limit the extent of the rotor strain while meeting end controlled variable demand. In electric power plant turbines, rapid starting of the turbine is accomplished by directing operation toward steady maximum allowable rotor surface strain during startup while limiting rotor strain below a maximum limit. Operation during load transients is also controlled to achieve rapid load change under conditions of maximum allowable rotor strain.

In its preferred form, the system of this invention utilizes a general purpose programmed digital computer for determining end variable control actions during transient and steady state operation with dynamic constraints computed as a function of heat flow in one or more predetermined steam flow regions. The method of this invention may be used in conjunction with the method of operation disclosed in the Berry patent for providing control limits based upon rotor strain, bore loading and easing strain whereby, when any of such limits is exceeded, operation of the heat flow control provided by the invention can be over-ridden.

It is therefore an object of this invention to provide a novel method and system of steam turbine operation with greater accuracy, efficiency and economy, and improved performance characteristics.

It is a more specific object of this invention to provide a novel steam turbine system and method of operating same with real time control of heat flow to provide changes in turbine operation more responsive to present and future thermal stress and strain in monitored turbine regions so as to provide better turbine control resulting in more efficient electric power generation, and enabling more efficient turbine operation within dynamic constraints.

It is another object of this invention to provide automatic means for generating 'a representation of heat flow to at least one turbine region, and to control turbine operation as a function of such heat flow representation in a manner so as to maintain such heat flow at a level corresponding to maximum allowable temperature-induced (thermal) stress in such region.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic diagram of a large electric power plant steam turbine supplied with steam by a steam generating system and operated in association with certain sensor and control devices in accordance with the principles of the'invention.

FIG. 2 shows a schematic diagram of a programmed digital computer control system operable with the steam turbine and its associated devices shown in FIG. 1 in accordance with the principles of the invention.

FIG. 3 shows an enlarged portion of a longitudinal section through a high pressure section of the steam turbine of FIG. 1 and certain sensor devices placed therein.

FIG. 4 shows a control logic flow diagram employed in part of an overall programming system which operates the computer of FIG. 2 to control turbine operation in accordance with the principles of the invention.

FIG. 5 shows a more detailed flow diagram of a portion of the diagram of FIG. 4.

FIG. 6 shows a simplified block diagram illustrating the alternate procedure of calculating a plurality of differential heat flow representations corresponding to different turbine regions, and determining the lowest of same, which low representation is used to develop the system control signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT More specifically, there is shown in FIG. 1 a large single reheat steam turbine 10 constructed in a wellknown manner and operated and controlled in accordance with the principles of the invention as part of a fossil fuel fired electric power plant 12. Other types of steam turbines, such as extraction turbines, reactor turbines, back pressure turbines, etc. can also be controlled in accordance with the principles of the invention.

The turbine 10 is provided with a single output shaft 14 which drives a conventional large alternating current generator 16 to produce three phase (or other phase) electric power as measured by a conventional power detector 18. Typically, the generator 16 is connected (not shown) through one or more breakers (not shown) per phase to a large electric power network and when so connected causes the turbogenerator arrangement to operate at synchronous speed under steady state conditions. Under transient electric load change conditions, system frequency may be affected and conforming turbogenerator speed changes would result. At synchronism, power contribution of the generator 16 to the network is normally determined by the turbine steam flow which in this instance is supplied to the turbine 10 at substantially constant throttle pressure.

In this case, the turbine 10 is of the multistage axial flow type and includes a high pressure section 20, an intermediate pressure section 22 and a low pressure section 24. Each of these turbine sections may include a plurality of expansion stages provided by stationary vanes and an interacting bladed rotor connected to the shaft 14. In other applications, turbines operated in accordance with the present invention can have other forms with more or fewer sections tandemly connected to one shaft or compoundly coupled to more than one shaft.

The constant throttle pressure steam for driving the turbine 10 is developed by a steam generating system 26 which is provided in the form of a conventional drum type boiler operated by fossil fuel such as pulverized coal or natural gas. From a generalized standpoint, the present invention can also be applied to steam turbines associated with other types of steam generating systems such as nuclear reactor and once through boiler systems.

The turbine 10 in this instance is further of the double ended steam chest type, and turbine inlet steam flow is directed through a plurality of throttle valves and a plurality of governor valves designated as inlet valves 25. Generally, the double ended steam chest type and other steam chest types such as the single ended steam chest type or the end bar lift type may involve varying numbers and/or arrangements of throttle valves. More detailed description on a particular throttle and governor valve arrangement is presented in the aforementioned Birnbaum and Giras copending application.

The preferred turbine startup method is to (I) raise the turbine speed from the turning gear speed of about 2 rpm. to about 80 percent of the synchronous speed under throttle valve control and then (2) transfer to governor valve control and raise the turbine speed to the synchronous value, close the power system breaker(s) and meet the load demand. On shut-down, similar but reverse practices are involved. Other transfer practices can be employed, but it is unlikely that transfer would ever be made at a loading point about 40 percent rated loading because of throttling efficiency considerations.

After the steam has coursed past the first stage im- 'pulse blading to the last stage reaction blading of the high pressure section 20, it is directed to a reheater system 28 which is associated with the boiler 26. In practice, the reheater system 28 might typically include a pair of parallel connected reheaters coupled to the boiler 26 in heat transfer relation as indicated by the reference character 29 and associated with opposite sides of the turbine casing.

With a raised enthalpy level, the reheated steamflows from the reheater system 28 through the intermediate pressure turbine section 22 and the low pressure turbine section 24. From the latter, the vitiated steam is exhausted to a condenser 32 from which water flow is directed (not indicated) back to the boiler 26. To con trol the flow of reheat steam, reheat valves 33 are provided and these include one or more normally open check or stop valves and one or more intercept valves operable to provide reheat steam flow cutback modulation under turbine overspeed conditions.

In the typical fossil fuel drum type boiler steam generating system, the boiler control system controls boiler operations so that steam throttle pressure is held substantially constant. A throttle pressure detector 38 of suitable conventional design measures the steam throt tle pressure to provide assurance of substantially constant throttle pressure supply, and, if desired as a programmed computer protective system override control function, turbine control action can be adapted to throttle pressure control as well as or in place of speed and/or load control if the throttle pressure falls outside predetermined constraining safety and turbine condensation protection limits. An impulse chamber steam pressure detector 40 develops signals for use in programmed computer control of turbine load and ultimately power plant electrical load.

Respective hydraulically operated valve actuators indicated by the reference character 42 are provided for the throttle and governor inlet valves 25. Hydraulically operated actuators indicated by the reference characters 44 are also provided for the reheat stop and intercept valves 33 A computer sequenced and monitored high pressure fluid supply 46 provides the controlling fluid for actuator operation of the valves 25 and 33. A computer supervised lubricating oil system (not shown) is separately provided for turbine plant lubricating requirements.

The respective actuators 42 and 44 are of conventional construction, and the actuators 42 and the actuators 44 associated with the intercept valves are operated by respective stabilizing position controls indicated by the reference characters 48 and 50. These controls each include a conventional position error feedback operated analog controller (not indicated) which drives a suitable known actuator servo valve (not indicated) in the well-known manner. Reheat intercept 6 valve position control is imposed typically only when reheat steam flow cutback modulation is required. Stop valve operation requires no feedback position control and instead is manually or computer directed with conventional trip or other suitable emergency operation.

Since turbine power is proportional to steam flow under the assumed controlled condition of substantially constant steam throttle pressure, steam valving position is controlled to produce control over steam flow as an intermediate variable and over turbine speed and/or load as an end controlled variable(s). Actuator operation provides the steam valve positioning, and respective valve position detectors P'DIV and PDRV are provided to generate respective valve position feedback signals for developing position error signals to be applied to the respective position controls 48 and 50. The

position detectors are provided in suitable conventional form, for example they can make conventional use of linear variable differential transformer operation in generating negative position feedback signals for algebraic summing with respective position setpoint signals SP in developing the respective input position error signals.

The combined position control, hydraulic actuator, valve position detector element and other miscellaneous devices (not shown) form a local hydraulic-electrical analog valve position control loop for each throttle and governor inlet steam valve. The position setpoints SP are computer determined and supplied to the respective local loops and updated on a periodic basis. Setpoints SP are also determined for the intercept valve controls. A more complete general background description of electrohydraulic steam valve positioning and hydraulic fluid supply systems for valve actuation is presented in the aforementioned Birnbaum and Noyes paper.

A speed detector 52 is provided to determine the turbine shaft speed for speed control, for centrifugal stress determination and turbine constraint operation, for frequency participation control purposes, and preferably also for rotor surface heat transfer conductance computation associated with rotor thermal strain control. The speed detector 52 can for example be in the form of a reluctance pickup (not shown) magnetically coupled to a notched wheel (not shown) on the turbogenerator shaft 14. The process sensor equipment further includes an impulse chamber steam temperature detector 54 and easing temperature detectors 56, all of which are employed in programmed computer loading andthermal strain determination as subsequently described more fully. Analog and/or pulse signals produced by the speed detector 52, the power detector 18, the pressure detectors 38 and 40, the temperature detectors 54 and 56, the valve position detectors PDIV and PDRV and other sensors (not specifically shown) and status contacts (not specifically shown) are all applied to a digital computer control system 60 (FIG. 2) which provides turbine steady state and transient operation control on an on line real time basis and further provides system monitoring, sequencing, supervising, alarming, display and logging functions.-

The programmed digital computer control system 60 operates the turbine 10 with improved dynamic performance characteristics, and can include conventional hardware in the form of a central processor 62 and associated input/output interfacing equipment such as that sold by Westinghouse Electric Corporation and described in detail in Westinghouse Engineer," May, 1970, Volume 30, No. 3, pages 88 through 93. As will be apparent from the description hereinbelow, the control system of this invention may utilize, for performing the indicated calculations, any general purpose programmable computer having real time capability, in combination with the control apparatus illustrated in FIG. 1 and the required interface equipment, or equivalents thereof, as illustrated in FIG. 2. Also, it is to be understood that special purpose analog computer apparatus may be utilized for making the specific calculations required to practice this invention in controlling the operation of any particular turbine.

The interfacing equipment for the computer processor 62 includes a conventional contact closure input system 64 which scans contact or other similar signals representing the status of various plant and equipment conditions. Such contacts are generally indicated by the reference character 66 and might typically be contacts of mercury wetted relays (not shown) which are operated by energization circuits (not shown) capable of sensing the predetermined conditions associated with the various system devices. Status contact data is used in interlock logic functioning in control or other programs, protection and alarm system functioning, programmed monitoring and logging and demand logging, functioning of a computer executed manual supervisory control 68, etc.

The contact closure input system 64 also accepts digital load reference signals as indicated by the reference character 70. The load reference 70 can be manually set or it can be automatically supplied as by an economic dispatch computer (not shown). In the load control mode of operation, the load reference 70 defines the desired megawatt generating level and the computer control system 60 operates the turbine 10 to supply the power generation demand.

Input interfacing is also provided by a conventional analog input system 72 which samples analog signals from the plant 12 at a predetermined rate such as 15 points per second for each analog channel input and converts the signal samples to digital values for computer entry. The analog signals are generated by the power detector 18, the impulse pressure detector 40, the valve position detectors PDIV and PDRV, the temperature detectors 54 and 56, and miscellaneous analog sensors 74 such as the throttle pressure detector 38 (not specifically shown in FIG. 2), various steam flow detectors, other steam temperature detectors, miscellaneous equipment operating temperature detectors, generator hydrogen coolant pressure and temperature detectors, etc; A conventional pulse input system 76 provides for computer entry of pulse type detector signals such as those generated by the speed detector 52. The computer counterparts of the analog and pulse input signals are used in control program execution, protection and alarm system functioning, programmed and demand logging, etc.

Information input and output devices provide for computer entry and output of coded and noncoded information. These devices include a conventional tape reader and printer system 78 which is used for various purposes including for example program entry into the central processor core memory. A conventional teletypewriter system 80 is also provided and it is used for purposes including for example logging printouts as indicated by the reference character 82. Alphanumeric and/or other types of displays 81, 83 and 85 are used to 8 communicate current rotor strain, accumulated rotor strain fatigue, and other information.

A conventional interrupt system 84 is provided with suitable hardware and circuitry for controlling the input and output transfer of information between the computer processor 62 and the slower input/output equipment. Thus, an interrupt signal is applied to the processor 62 when an input is ready for entry or when an output transfer has been completed. In general, the central processor 62 acts on interrupts in accordance with a conventional executive program. in some cases, particular interrupts are acknowledged and operated upon without executive priority limitations.

Output interfacing is provided for the computer by means of a conventional contact closure output system 86 which operates in conjunction with a conventional analog output system 88 and with a valve position control output system 90. A manual control 92 is coupled to the valve position control output system and is operable therewith to provide manual turbine control during computer shutdown and other desired time periods.

Certain computer digital outputs are applied directly in effecting program determined and contact controlled control actions of equipment including the high pressure valve fluid and lubrication systems as indicated by the reference character 87, alarm devices 94 such as buzzers and displays, and predetermined plant auxiliary devices and systems 96 such as the generator hydrogen coolant system. Computer digital information outputs are similarly applied directly to the tape printer and the teletypewriter system and the display devices 81, 83 and 85.

Other computer digital output signals are first converted to analog signals through functioning of the analog output system 88 and the valve position control output system 90.. The analog signals are then applied to the auxiliary devices and systems 96, the fluid and lubrication systems 87 and the valve controls 48 and 50 in effecting program determined control actions. The respective signals applied to the steam valve controls 48 and 50 are the valve position setpoint signals SP to which reference has previously been made.

Reference is made to MG. 3 for a detailed showing of the more significant portions of an illustrative structural arrangement for the turbine high pressure section 20 and for the preferred turbine temperature sensor arrangement associated therewith. The turbine high pressure section 20 includes a casing or cylinder wall 100 within which a rotor 182 is supported for rotation. Casing strain at predetermined casing locations is based on conventional outer and inner wall temperature thermocouple probes 104 and 106 which form a part of the casing temperature detectors 56.

A suitable steam temperature sensor (not specifically shown but included as a part of the analog sensors 78) can also be employed in the intermediate pressure section 22, such as in the inlet steam pipe but preferably in the IP inlet steam chamber (not shown). lP steam temperature data is used in the computation of rotor bore-thermal stress in the intermediate pressure section 22.

Steam enters the turbine 10 through a plurality of peripherally disposed inlets 108 and associated nozzle blocks 105, and the steam is directed through a velocity compounded impulse control stage including two rows of rotor impulse blades 107 and 109 and one row of stationary blades 111 into an impulse chamber 110. As indicated by the flow arrows, the steam then reverses 9 its flow direction and passes through reaction blading 112 in the successive stages of the high pressure sec tion. A conventional thermocouple probe 114 is appropriately supported by the casing 100 to measure'the impulse chamber steam temperature.

Referring now to FIGS. 4 and 5, there are shown flow diagrams representing the manner of calculating the the more specific program steps charted in FIG. 5,-

constitute, for the preferred embodiment where a programmed digital computer is utilized, a sub-program which is a portion of a programming system employed to operate the computer system 60. Reference is made to the programming system disclosed in the Berry patent starting at column 12, line 50, and continuing to column 13, line 35, for an example of such a larger programming system. The improvement of this invention over the system of the Berry patent is focused in the use of the heat flow control sub-program of FIGS. 4 and 5, independent of or in conjunction with the sub-program of the Berry patent which derives limits of turbine operation as a function of rotor strain, casing strain and bore strain. It is to be understood that all or any specific portion of the functional operations illustrated in FIGS. 4 and 5 may be carried out by special purpose digital or analog calculating means or equivalent apparatus which provides the necessary real time capability. For operation with digital computer means, the Westinghouse W-2500 has the requisite capacity and is suitable for use as the central processor 62. In other cases, the Westinghouse Digital Electro- Hydraulic (DEH) Control System for large steam turbine generators may be utilized in practicing this invention.

Table 1 set forth below gives definitions for the symbols used in the flow charts of FIGS. 4 and 5. It is to be noted that some of the arithmetic operations are repre- Q,, Rotor heat flow limit. i.e., heat flow for maximum allowable rotor strain.

0 heat flow; Q (T, T H A, for rotor.

T, rotor surface temp. f (W,,-, T,, P,)

H heat transfer coefficient f (W SF) A rotor surface area T, first stage temp. (measured) T rotor volume average temp. (defined in the Berry patent) SF Steam Flow Rate (measured) W,- shaft speed (measured) P, first stage pressure (measured) DTO T T. a measure of rotor surface strain D'l'P predicted DTO DTO DDTO, where DDTO d(DTO)/dt, extrapolated rate of change of DTO.

W,, speed reference NR speed control signal g gain constants DNR d(NR)/dt. derivative of NR LDNR limited DNR GDNR gated DNR HRL high rate limit LRl. low rate limit DPTLIM limit of DPT DTOLIM limit of DTO INC increment signal INT integrated increment signal NDI. speed deceleration reference LR load control signal DLR derivative of LR LDLR limited DLR GDLR gated DLR LDL load deceleration reference L load (measured) L,, load reference T period between calculations Referring first to FIG. 4, block 310 represents the collective steps of calculating representations of DTO, DTP, Q, T and T. The input variables to block 310 are obtained as illustrated in FIGS. l3, with W T,, SF and P, being continuously monitored variables, while A is a constant which is stored and read (as by the tape reader 78) when called for by the sub-program. Reference is made to the Berry patent for a discussion of the calculations of T and T DTO is a measure of rotor surface strain, and DTP is a predicted value of DTO obtained by adding the first derivative of DTO to DTO.

In the preferred embodiment of this invention, the representation of Q, as calculated at block 310, is a representation of heat flow to the rotor surface. However, as set forth more fully hereinbelow, Q may represent calculated heat flow to other regions of the turbine, such as the rotor bore, or casing walls. The significance of the calculated heat flow term is that it represents not merely a present temperature condition at a given region, but represents the rate at which temperature and strain itself at such region is changing. In the case of heat flow to the rotor, Q (T, T H A,

I where H represents the heat transfer coefficient at the rotor surface, and takes into account surface film and all other considerations affecting heat transfer'from the steam to the rotor surface. Reference is made to the Berry patent for a discussion of the derivation of the formula for the heat transfer coefficient. As is seen, Q is a direct function of H, which in turn is a function of W and SF, measured quantities. Under startup conditions, H is primarily a function of W and under load control conditions, H is primarily a function of SF. By controlling speed at startup, Q is modulated, and when the turbine is under load control, Q is modulated by controlling steam flow. Thus, this system entails control of turbine operation so as to control heat flow, with heat flow being maintained at a level corresponding to optimum turbine performance.

At block 320, the computer means carries out the function of calculating the difference between Q, and Q, representing the difference between the heat flow limit corresponding to maximum allowable rotor strain and the present heat flow. Q, is the permissible heat flow for maximum stress under normal transient conditions. While generally treated as a constant, it is to be noted that Q, may be periodically recalculated to take into account the mode of control, centrifugal force loadings, or other considerations as noted below. An example of a normal transient condition is the condition of startup, or acceleration, where the temperature gradient through the rotor is substantially a constant, and under which conditions the heat transfer coefficient changes with speed in a manner such thatchanging speed causes a corresponding change in heat flow. A similar steady state transient condition exists where, at synchronous speed, the turbine is called upon to deliver an increasing load at a constant rate of increase, i.e., a ramp increase.

The difference representation 0,, Q, and referred to as DQ, is operated upon as shown in parallel blocks 330 and 351, by multiplication and integration respectively, and summed at 332 to produce a signal which is representative ofthe first derivative, designated as DNR, of the speed signal NR. The DNR signal is limited in step 350 by a high rate limit HRL and a low rate LRL, as discussed more fully hereinbelow in connection with FIG. 5. The DNR signal is gated, as shown at block 360, and is either passed or not passed corresponding to 1 1 limit controls derived from the DTO and DTP values, to produce a gated DNR signal referred to as GDNR.

Referring back to the output of block 310,-where are indicated the steps of calculating DTO'and signals, the DTO signal is operated upon in block '366 to determine the comparison of DTO and DTOLIM,a predetermined limit of DTO. The DTOLIM' signal represents the maximum value of calculated DTO permissible to maintain operation without rotor strain, and corresponds to the maximum permissible present value of rotor strain. Thus, while the control signal is being calculated in terms of heat flow, the comparison in block 366 comprises a direct check upon present strain. If DTO is greater than DTOLIM, a signal is derived, as indicated in block 362, to cause the gating operation as illustrated in block 360 to cause the GDNR signal to be zero, i.e., the DNR signalis multiplied by zero. If DTO is found to be less than DTOLIM, a further comparison is made at block 364, between DTP and DTPLIM. DTPLIM represents a limiting value of DTP, or the maximum value of predicted rotor strain. Similarly, if DTP is found to be greater than DTPLIM, a signal is developed at blocks 360 and 362 to cause the DNR signal to be reduced to zero, i.e., make GDNR equal to zero. If DTP is found to be less than or equal to DTPLIM, the DNR signal is gated straight through, i.e., multiplied by one, such that DGNR equals DNR.

The GDNR signal, being a gated representation of thefirst derivative of the speed control signal, is oper ated upon as shown in block 370. In the preferred embodiment, this operation comprises the step of integrating the GDNR signal to obtain an NR signal representing the speed control signal.

The speed control signal, as generated in the manner described, represents the basic command for controlling the speed of the turbine, as during startup, in order to bring turbine operation to the desired (synchronous) speed in the quickest time'and substantially at but not exceeding rotor strain limits. During a period of speed change, heat transfer from the impulse chamber steam to the rotor surface is assumed to be proportional to speed (since the heat transfer coefficient changes primarily as a function of speed), and thus in controlling speed the heat flow to the rotor is accordingly controlled. Further, and most importantly, since the calculated value of heat flow (Q) is a determinant of future strain, the turbine operation is controlled not merely to keep the present operation within safe limits (as done through the comparisons 364 and 366), but also to command changes in speed so as to optimally maintain operation at the desired stress limits. Stating it another way, the difference between operation under this invention and operation under prior art control systems is that of commanding operation to be maintained at maximum strain limits, as opposed to constraining operation within such limits.

It is to be understood that alternate methods may be employed to calculate the speed control representation NR on the basis of heat flow O. Other processing or limiting steps may be utilized in deriving the NR signal from the Q signal so long as loop stability is maintained; and be within the spirit and scope of this invention, the important feature being that the ultimately. derived control signal be based vupon the calculation of heat flow.

f. -.,-In';ac;tual practice, the control signal NR may be further operatedupon at speed/load control block 382, 'in ordcr to develop a desired valve position signal. For example, when in the speed control mode of operation, the NR signal may be processed as a function of W and/or W and when in the load control mode of operation, the load control signal LR may be further processed as a function of LS and/or LR. The processed signal is then operated upon at block 386 to determine digital output valve position values which are transmitted to valve position control 90. In addition, the digital output valve position valuesmay be modified by calcu lated maximum value signals from block 388, which calculations are referred to further hereinbelow.

Referringnow to FIG. 5, there is shown a flow chart for a specific computer subroutine for calculating the speed reference signal NR as a function of heat flow. The start of the subroutine is understood to be initiated periodically when the control calculations are carried out by digital computer, and at a rate sufficient to maintain real time control. Of course, if the computer means is comprised of analog circuitry, the calculations are performed continuously.

The subroutine is started at 401, and a determination is madeas to whether the system is in speed or load control. H, calculated at 405, is primarily a function of the rate at which the steam passes relative to the rotor. Under startup conditions, speed is relatively low, and the change in speed comprises substantially all of the relative change, such thatsteam flow itself is not a significant factor. Thus, H is presumed to be primarily a function of W (measured at 52). For example, the formula H k (W 1000), where k is a true constant, has been found to represent H under startup control. Conversely, under load control, the rotor speed is fixed at the synchronous value, and H is presumed to be primarily a function of SF (measured at 74).

After calculation of H, the calculation Q (T, T,,) H A is performed at step 410. As indicated previously, the T, variable is measured at the impulse chamber (see FIG. 3) and the value thus monitored by detector S4 is introduced through the analog input system 72. For digital computer processing, the analog signal is converted into digital form.

In the remainder of the flow diagram of FIG. 5, it is assumed, for illustration only, that the system is in speed control. In the preferred embodiment, the value ofQ is next subtracted from O as shown at block 320, to obtain a reference signal DQ, representing the difference between the value of heat flow at which turbine operation is maintained at maximum heat stress, and the present calculated heat flow. The DQ signal is next multiplied, as shown at block 330, by a gain function g,, to derive the signal designated as DNR(P), being the proportional component of DNR.

There is also shown in FIG. 5, within the section outlined by a dashed line, a parallel path for obtaining DNR(I), the integral component of DNR. This path comprises determining, as shown at 421, whether the value of Q Q is greater than or equal to zero, or less than zero. If the former is the case, a positive incremental signal INC, having a constant value of is devel-- oped at 423. If Q -Q is less than zero, a negative incremental signal having a constant yalue of V is developed as shown at 425. The incremental signal,

whether plus or minus, and designated INC, is then integrated by the trapezoidal function shown in 13 block 340, whereby the DNR signal is generated. To account for integrator windup in block 340, a limiting function (not shown) may also be incorporated. The DNR(P) and DRN(I) signals are summed at 332 to provide the DNR signal.

TlE DNR signal is limited by the functions illustrated within block 350. DNR is first compared with the high rate limit HRL, as shown at 431, representing a maximum allowable rate of increase of speed. If DNR is greater than or ,equal to HRL, a limited DNR signal designated as LDNR is derived which is equal to HRL, asshown at 434. If DNR is less than HRL, or within the high rate limit, the DNR signal is then compared with the low rate limit signal LRL, as shown at 433. If DRL is equal to or lower than LRL a minimum allowable rate of increase of speed, LDNR is set equal to LRL, as shown at 437. If DNR is greater than LRL (as well as being less than l-IRL), LDNR is set equal to DNR.

' The subroutine next performs the constraint checks indicated at 364, 366 in-FIG. 4. As shown at 440 in FIG. 5, DTO is examined to see if it is less than DTO- LIM, and DTP is examined to see if it is less than DTPLIM. If these conditions are met, the LDNR signal is integrated, by the trapezoidal integration function indicated at 450, similarly to the step illustrated at 340, 2

to derive the final NR signal. If the condition as shown at 440 is not met, the old value of NR, i.e., NR (1. 1), is provided as the NR signal.

The discussion of the preferred embodiment as set forth above has illustrated the practice of this invention in controlling turbine operation during startup, during which the control signal which is derived is used to position steam inlet valves so as to control turbine speed, the controlled speed in turn controlling heat flow in such a way as to cause operation to be substantially at the limit of allowable rotor stress. In practice, valve position control 90 controls the throttle valve (on-off), or valves, during the startup phase until the turbine speed reaches a predetermined speed short of full speed, e.g., about 3,000 r.p.m. for a synchronous speed of 3,600 r.p.m. At this point, control is transferred to the control or governor valve, or valves, which modulate steam input to any value between full' off and full on, for bringing speed up to the full synchronous value.

It is also to be noted, as indicated previously, that this invention can bepracticed in controlling operation changes in response to different load demands made upon the turbine. After the turbine has obtained synchronous speed, where it is maintained, an increased demand for load presents a steady state transient condition calling for increased turbine output, similar to the startup condition except that while speed is maintained constant, load output (MW) is increased. In this mode of operation, the Q reference signal which is inputted at step 320 may be the same'value, or may be another value chosen for load mode operation. The difference signal, DQ, represents the difference between calculated (present) heat flow and the reference heat flow under load conditions, which DQ signal is operated upon in the same manner as when in the startup mode. At step 350, the limits HRL and LRL, stored in memory, represent maximum rates of increase of load. Similarly, the value of DO after having been multiplied by the gain function at block 330, is referred to as DLR; the limited value of DLR is referred to as LDLR; and the gated value of LDLR is designated as GDLR. The final signal generated after the integrate step at 370 is referred to as LR, the load '14 control reference signal. The above signals, which are developed under load control operation, are shown in parentheses in FIG. 4.

The basic technique of this invention may also be applied to casing strain and/or bore strain, by calculating (at block 310-A) heat flow to the casing (Q and heat flow to the rotor bore (designated Q As shown in FIG. 6, each of these values may be compared (at 320-A) with a corresponding reference (Q Q etc.) and respective values of DO (normalized) may be obtained which are compared at 465 to determine the low limit DQ representation. The lowest value representation of DO is chosen as controlling, and is thereafter processed to obtain the desired control signal. Thus, system operation may be controlled on the basis of heat flow to that turbine region wherein heat flow is closest to the limit for maintaining maximum strain.

From the above, it is seen that there is disclosed an improved rotor stress control system based upon heat flow calculations. The system of operation of this invention may be used independently, or it may be used in conjunction with further calculations of operating constraints based upon calculated fatigue, bore thermal 5 loadings, bore centrifugal loadings, casing wall strain and the like. Referring again to the Berry patent, a turbine rotor loading and thermal strain constraint subroutine is shown, in block diagram form, in FIGS. 9 and 10 of that patent. This subroutine is an illustration 0 of a manner of calculating further constraints which may be imposed upon the speed or load control signal. It is to be noted that such subroutine provides for signals which may be used as constraints on the change in valve position under either startup or load control,

5 which constraints are based upon calculated bore thermal loadings, bore centrifugal loadings, and casing strain. In addition, the fatigue damage per cycle may be calculated, and a tally maintained of accumulated fatigue damage, permitting more accurate rotor plastic 0 strain fatigue supervision and/or control. The signals produced by such subroutine may be used to modify the Q signal, or may be processed at block 388 to determine maximum values of steam valve positions. In this manner, while the system is under the heat flow 5 control of this invention, it may also be subjected to limit control on the basis of calculated present loadings and strain at various regions of the turbine.

The application of the invention as described hereinabove produces generally improved steam turbine operation and, more specifically, it provides for quicker and more accurate control system functioning as compared to prior art systems based upon present rotor stress calculations. The foregoing description has been presented to illustrate the principles of the invention and it is to be understood that the means for carrying out the various functions performed in the practice of this invention are illustrative of the preferred embodiment. Accordingly, it is desired that the invention not be limited by the embodiment described, but, rather that it be afforded a scope consistent with its broad principles.

It is noted that in the illustration of the preferred embodiment of this application reference is made to controlling turbine operation as a function of the thermal condition of ,a specified portion of the entire turbine system. As is well known, however, there may be instances where more than one turbine location may limit operation. For example, a large steam turbine is normally composed of HP, IP and LP sections, and

thermal stress in any of these sections may be cause for limiting the speed or loading rate. Thus, the inputs to difference box 320-A (FIG. 6) may include Q signals from any or all of the turbine sections, such that low limit select 465 provides that system operation be controlled by the heat flow representation of that turbine section having the most limiting thermal condition.

I claim:

1. An improved steam turbine system comprising:

a. A steam turbine having a portion subject to thermal stress when said turbine is in operation;

b. means for generating a heat flow representation of present heat flow to said portion from a predetermined region in heat transfer relation to said portion; and

c. means for controlling operation of said steam turbine as a function of said heat flow representation, whereby said thermal stress is controlled.

2. The improved steam turbine system as described in claim 1, wherein said turbine portion is the turbine rotor, and said heat flow representation represeents heat flow to the surface of said rotor.

3. The improved steam turbine system as described in claim 2, comprising means for generating a difference representation representing the difference between a reference heat flow and said rotor heat flow, and means for generating a control signal as a function of said difference representation, and wherein said controlling means controls operation of said steam turbine as a function of said control signal.

4. The improved steam turbine system as described in claim 3 comprising means for detecting steam temperature and steam pressure in a predetermined turbine region in heat transfer relation with said rotor and for generating therefrom a steam temperature signal and a steam pressure signal, and wherein said means for generating a representation of heat flow to said rotor performs the function of calculating heat flow to the rotor on the basis of said steam temperature and steam pressure signals.

5. The improved steam turbine system as described in claim 4, comprising means for determining rotor speed and generating a rotor speed signal, and wherein said means for generating said heat flow representation makes said heat flow calculation as a function of rotor speed when said system is controlling the speed of said turbine.

6. The improved steam turbine system as described in claim 5, comprising means for determining steam flow in said region and generating a steam flow signal, and wherein said means for generating a heat flow representation makes said heat flow calculation as a function of said steam flow signal when said system is controlling turbine load.

7. The improved steam turbine system as described in claim 3, wherein said means for generating a control signal incorporates means for limiting the rate of increase of said heat flow difference representation, and means for limiting said control signal as a function of present rotor surface strain and predicted rotor surface strain.

8. The improved steam turbine system as described in claim 5, wherein said means for generating a control signal generates a speed control signal and said means for controlling operation controls turbine speed when the system is in speed control operation.

9. The improved steam turbine system as described in claim 6, wherein said means for generating a control signal generates a steam flow signal and said means for controlling operation controls turbine steam flow when said turbine system is in load control operation.

10. The improved steam turbine system as described in claim 1, wherein said steam turbine has a plurality of portions subject to thermal stress, and comprising a plurality of means for generating representations of heat flow to respective ones of said portions and means for determining a controlling one of said heat flow representations as a function of which said steam turbine is controlled.

11.. The improved steam turbine system as described in claim 10, wherein one of said additional portions is the turbine casing.

12. The improved steam turbine system as described in claim 1, wherein said turbine portion is the turbine casing, and said heat flow representation represents heat flow to said casing.

13. A control system for a steam turbine comprising:

a. means for generating a representation of present time rate of change of temperature at the turbine rotor due to heat flow to said rotor from a predetermined region in heat transfer relation thereto;

b. means for controlling steam flow to said turbine as a function of said heat flow representation.

14. The control system for a steam turbine as described in claim 13, wherein said means for controlling said steam flow controls said steam flow so as to change turbine speed.

15. The control system for a steam turbine as described in claim 13, wherein said means for controlling steam flow controls said steam flow so as to change the load carried by said steam turbine.

16. The control system for a steam turbine as described in claim 14, wherein said means for controlling steam flow includes steam valve means positioned to determine steam flow through said turbine so as to control turbine speed.

17. The control system for a steam turbine as described in claim 15, wherein said means for controlling steam flow includes steam valve means positioned to determine steam flow so as to control load delivered by the turbine.

1.8. A control system for controlling the operation of a steam turbine. comprising:

a. means for determining the temperature difference between the steam temperature in a predetermined turbine region in heat transfer relation with the turbine rotor surface and the temperature of such turbinerotor surface;

b. means for determining the rotor speed and steam flow through the predetermined region;

c. means for generating a turbine operating representation as a function of said temperature difference, rotor speed and steam flow; and

d. steam valve means operated by said operating representation for controlling steam flow to said turbine as a function of said operating representation.

19. The control system as described in claim 18 wherein said operating representation is a function of heat flow to the rotor surface, and comprising means for generating a heat flow limit representation as a function of a predetermined turbine thermal condition and for limiting the operation of said valve means in accordance with said limit.

20. The control system as described in claim 18, comprising a general purpose programmed digital com- 17 puter which performs the following functions:

a. generating a representation of the heat transfer coefficient at said rotor surface;

b. generating a representation of present heat flow to said rotor surface as a function of said heat transfer coefficient and said temperature difference;

c. generating a representation of the heat flow difference between a reference heat flow corresponding to maximum heat flow for allowable rotor strain and present heat flow; and

d. generating said operating representation as a function of said difference representation.

21. The control system as described in claim 20,

wherein said operating representation is a speed representation.

22. The control system as described in claim 20, wherein said operating representation is a load representation.

- 23. An improved method for operating a steam turbine, which turbine when operating has one or more portions thereof subject to thermal stress, comprising:

a. generating a heat flow signal representing present heat flow to one of said portions from a predetermined region separate from and in heat transfer relation to said one portion;

b. generating a control signal as a function of said heat flow signal; and

c. controlling the operation of said steam turbine as a function of said control signal, thereby controlling thermal stress in said one portion.

24. The improved method for operating a steam turbine as described in claim 23, wherein said one portion is the turbine rotor, and the step of generating said control signal comprises generating a difference signal representing the difference between a reference heat flow to the rotor corresponding to maximum allowable rotor strain and the heat flow to the rotor.

25. The improved method for operating a steam turbine as described in claim 24 wherein the step of generating said heat flow signal comprises determining steam temperature and pressure in a predetermined turbine region in heat transfer relation with the surface of said rotor; calculating heat flow to the rotor as a function of said determined steam temperature and pressure; and controlling turbine operation as a function of said calculated rotor heat flow.

26. The improved method for operating a steam turbine as described in claim 25, comprising determining rotor speed and calculating heat flow to the rotor as a function also of rotor speed, and controlling turbine speed as a function of the calculated rotor heat flow.

27. The improved method for operating a steam turbine as described in claim 25, comprising determining steam flow in said turbine region and calculating heat flow to the rotor as a function of steam flow, and controlling turbine load as a function of the calculated rotor heat flow.

28. The method for operating a steam turbine as described in claim 25, comprising limiting said control signal as a function of stress at other portions of said turbine.

29. The improved method for operating a steam turbine as described in claim 23, comprising generating a plurality of heat flow signals representing heat flow to a plurality of respective turbine portions subject to heat stress; selecting a limiting one of said heat flow signals; and generating said control signal as a function of said selected heat flow signal.

30. The improved method for operating a steam turbine as described in claim 23, wherein said one portion is the turbine casing.

31. The improved method for operating a steam turbine as described in claim 29, wherein one of said turbine portions is the turbine casing.

32. The improved method for operating a steam turbine as described in claim 23, further comprising limit signals representing rotor surface strain, accumulated fatigue damage, bore loadings, and easing strain, and limiting said control signal on the basis of one of said limit signals.

33. A method for operating a steam turbine comprisin i. determining steam temperature in a predetermined turbine region separate from and in heat transfer relation with a preselected turbine rotor portion;

b. generating a representation of present heat flow from said region to said turbine rotor portion as a predetermined function of said determined steam temperature; and

c. controlling the steam turbine speed as a function of said heat flow representation and continuously controlling the heat flow to said rotor substantially corresponding to maximum allowable rotor strain.

34. The method as described in claim 33, wherein the step of controlling the steam turbine speed comprises:

a. generating a difference signal representing the difference between a reference heat flow corresponding to maximum allowable rotor strain, and heat flow to the rotor; and

b. generating a control signal as a function of said difference signal, and controlling said steam turbine speed by said control signal.

35. The method as described in claim 34- further comprising generating limit signals representing limiting values of conditions selected from the group consisting of rotor surface strain, bore strain and casing strain, and limiting said control signal on the basis of one of said limit signals.

36. A method for controlling a steam turbine comprising:

a. generating a representation of current heat flow to the turbine rotor from a predetermined region in heat transfer relation to said rotor; and

b. continuously controlling steam flow to said turbine as a function of said current heat flow representation.

37. The method for control of a steam turbine as described in claim 36, comprising controlling said steam flow so as to change turbine speed.

38. The method for control of a steam turbine as described in claim 36, comprising controlling said steam flow so as to change the load carried by said steam turbine.

39. The method of controlling a steam turbine as described in claim 36, wherein said heat flow representation is generated as a function of the impulse chamber steam temperature (T,), the rotor surface temperature (T the heat transfer coefficient at the rotor surface (H), and the rotor surface area (A).

40. The method of controlling a steam turbine as described in claim 39, wherein H is determined as a function of rotor speed and steam flow past the rotor.

41. The method of controlling a steam turbine as described in claim 40, comprising determining at least one other turbine thermal condition, and limiting said steam flow to said turbine when said thermal condition 19 exceeds a predetermined limit.

42. The method of controlling a steam turbine as described in claim 41, wherein said thermal condition is rotor surface strain.

43. The method of controlling a steam turbine as described in claim 41, wherein said thermal condition is casing strain.

44. A method for controlling steam turbine operation using a digital computer comprising:

a. determining the temperature difference between the steam temperature in a predetermined turbine region in heat transfer relation with the turbine rotor surface and the temperature of such turbine rotor surface;

b. determining the rotor speed and steam flow through said predetermined region;

c. generating, with a general purpose programmed digital computer, a turbine operating representation as a function of said temperature difference, rotor speed and steam flow; and

d. controlling steam flow to said turbine as a function of said operating representation.

45. The digital computer control method as described in claim 44 wherein said operating representation is a function of heat flow to the rotor surface, and further comprising the step of generating a heat flow limit representation as a function of a predetermined turbine thermal condition and limiting said turbine steam flow in accordance with said heat flow limit.

46. The digital computer control method as described in claim 44, comprising performing the following steps with said general purpose programmed digital computer:

a. generating a representation of the heat transfer coefficient at said rotor surface;

b. generating a representation of present heat flow to said rotor surface as a function of said heat transfer coefficient and said temperature difference;

c. generating a representation of the heat flow difference between a reference heat flow corresponding to maximum heat flow for allowable rotor strain, and present heat flow; and

d. generating said operating representation as a func' tion of said difference representation.

47. The digital computer control method as described in claim 46, wherein said operating representation is a speed representation.

48. A digital computer control method as described in claim 46, wherein said operating representation is a load representation.

49. An improved steam turbine system comprising:

a. a steam turbine having a portion subject to thermal stress when said turbine is in operation;

b. means for generating a first representation of a first predetermined temperature condition at said portion;

c. means for generating a second representation of a second predetermined temperature condition at a predetermined temperature region in heat transfer relation with said portion;

d. means for utilizing said first and second representations for generating a representation of heat flow to said portion as a function of said first and second temperature conditions; and

e. means for continuously controlling operation of said steam turbine as a function of said heat flow representation, whereby said thermal stress is controlled.

50. An improved steam turbine system comprising:

a. a steam turbine having a portion subject to thermal stress when said turbine is in operation;

b. means for generating a first representation of a first predetermined temperature condition at said portion;

c. means for generating a second representation of a second predetermined temperature condition at a predetermined temperature region in heat transfer relation with said portion;

d. means utilizing said first and second representa-- tions for generating a representation of the rate of change of said first predetermined temperature condition; and

e. means for controlling operation of said steam turbine as a function of said rate of change representation, whereby said thermal stress is controlled. 

1. An improved steam turbine system comprising: a. A steam turbine having a portion subject to thermal stress when said turbine is in operation; b. means for generating a heat flow representation of present heat flow to said portion from a predetermined region in heat transfer relation to said portion; and c. means for controlling operation of said steam turbine as a function of said heat flow representation, whereby said thermal stress is controlled.
 2. The improved steam turbine system as described in claim 1, wherein said turbine portion is the turbine rotor, and said heat flow representation represeents heat flow to the surface of said rotor.
 3. The improved steam turbine system as described in claim 2, comprising means for generating a difference representation representing the difference between a reference heat flow and said rotor heat flow, and means for generating a control signal as a function of said difference represeNtation, and wherein said controlling means controls operation of said steam turbine as a function of said control signal.
 4. The improved steam turbine system as described in claim 3 comprising means for detecting steam temperature and steam pressure in a predetermined turbine region in heat transfer relation with said rotor and for generating therefrom a steam temperature signal and a steam pressure signal, and wherein said means for generating a representation of heat flow to said rotor performs the function of calculating heat flow to the rotor on the basis of said steam temperature and steam pressure signals.
 5. The improved steam turbine system as described in claim 4, comprising means for determining rotor speed and generating a rotor speed signal, and wherein said means for generating said heat flow representation makes said heat flow calculation as a function of rotor speed when said system is controlling the speed of said turbine.
 6. The improved steam turbine system as described in claim 5, comprising means for determining steam flow in said region and generating a steam flow signal, and wherein said means for generating a heat flow representation makes said heat flow calculation as a function of said steam flow signal when said system is controlling turbine load.
 7. The improved steam turbine system as described in claim 3, wherein said means for generating a control signal incorporates means for limiting the rate of increase of said heat flow difference representation, and means for limiting said control signal as a function of present rotor surface strain and predicted rotor surface strain.
 8. The improved steam turbine system as described in claim 5, wherein said means for generating a control signal generates a speed control signal and said means for controlling operation controls turbine speed when the system is in speed control operation.
 9. The improved steam turbine system as described in claim 6, wherein said means for generating a control signal generates a steam flow signal and said means for controlling operation controls turbine steam flow when said turbine system is in load control operation.
 10. The improved steam turbine system as described in claim 1, wherein said steam turbine has a plurality of portions subject to thermal stress, and comprising a plurality of means for generating representations of heat flow to respective ones of said portions and means for determining a controlling one of said heat flow representations as a function of which said steam turbine is controlled.
 11. The improved steam turbine system as described in claim 10, wherein one of said additional portions is the turbine casing.
 12. The improved steam turbine system as described in claim 1, wherein said turbine portion is the turbine casing, and said heat flow representation represents heat flow to said casing.
 13. A control system for a steam turbine comprising: a. means for generating a representation of present time rate of change of temperature at the turbine rotor due to heat flow to said rotor from a predetermined region in heat transfer relation thereto; b. means for controlling steam flow to said turbine as a function of said heat flow representation.
 14. The control system for a steam turbine as described in claim 13, wherein said means for controlling said steam flow controls said steam flow so as to change turbine speed.
 15. The control system for a steam turbine as described in claim 13, wherein said means for controlling steam flow controls said steam flow so as to change the load carried by said steam turbine.
 16. The control system for a steam turbine as described in claim 14, wherein said means for controlling steam flow includes steam valve means positioned to determine steam flow through said turbine so as to control turbine speed.
 17. The control system for a steam turbine as described in claim 15, wherein said means for controlling steam flow includes steam valve means positioned tO determine steam flow so as to control load delivered by the turbine.
 18. A control system for controlling the operation of a steam turbine, comprising: a. means for determining the temperature difference between the steam temperature in a predetermined turbine region in heat transfer relation with the turbine rotor surface and the temperature of such turbine rotor surface; b. means for determining the rotor speed and steam flow through the predetermined region; c. means for generating a turbine operating representation as a function of said temperature difference, rotor speed and steam flow; and d. steam valve means operated by said operating representation for controlling steam flow to said turbine as a function of said operating representation.
 19. The control system as described in claim 18 wherein said operating representation is a function of heat flow to the rotor surface, and comprising means for generating a heat flow limit representation as a function of a predetermined turbine thermal condition and for limiting the operation of said valve means in accordance with said limit.
 20. The control system as described in claim 18, comprising a general purpose programmed digital computer which performs the following functions: a. generating a representation of the heat transfer coefficient at said rotor surface; b. generating a representation of present heat flow to said rotor surface as a function of said heat transfer coefficient and said temperature difference; c. generating a representation of the heat flow difference between a reference heat flow corresponding to maximum heat flow for allowable rotor strain and present heat flow; and d. generating said operating representation as a function of said difference representation.
 21. The control system as described in claim 20, wherein said operating representation is a speed representation.
 22. The control system as described in claim 20, wherein said operating representation is a load representation.
 23. An improved method for operating a steam turbine, which turbine when operating has one or more portions thereof subject to thermal stress, comprising: a. generating a heat flow signal representing present heat flow to one of said portions from a predetermined region separate from and in heat transfer relation to said one portion; b. generating a control signal as a function of said heat flow signal; and c. controlling the operation of said steam turbine as a function of said control signal, thereby controlling thermal stress in said one portion.
 24. The improved method for operating a steam turbine as described in claim 23, wherein said one portion is the turbine rotor, and the step of generating said control signal comprises generating a difference signal representing the difference between a reference heat flow to the rotor corresponding to maximum allowable rotor strain and the heat flow to the rotor.
 25. The improved method for operating a steam turbine as described in claim 24 wherein the step of generating said heat flow signal comprises determining steam temperature and pressure in a predetermined turbine region in heat transfer relation with the surface of said rotor; calculating heat flow to the rotor as a function of said determined steam temperature and pressure; and controlling turbine operation as a function of said calculated rotor heat flow.
 26. The improved method for operating a steam turbine as described in claim 25, comprising determining rotor speed and calculating heat flow to the rotor as a function also of rotor speed, and controlling turbine speed as a function of the calculated rotor heat flow.
 27. The improved method for operating a steam turbine as described in claim 25, comprising determining steam flow in said turbine region and calculating heat flow to the rotor as a function of steam flow, and controlling turbine load as a function of the calculated rotor heat flow.
 28. The method fOr operating a steam turbine as described in claim 25, comprising limiting said control signal as a function of stress at other portions of said turbine.
 29. The improved method for operating a steam turbine as described in claim 23, comprising generating a plurality of heat flow signals representing heat flow to a plurality of respective turbine portions subject to heat stress; selecting a limiting one of said heat flow signals; and generating said control signal as a function of said selected heat flow signal.
 30. The improved method for operating a steam turbine as described in claim 23, wherein said one portion is the turbine casing.
 31. The improved method for operating a steam turbine as described in claim 29, wherein one of said turbine portions is the turbine casing.
 32. The improved method for operating a steam turbine as described in claim 23, further comprising limit signals representing rotor surface strain, accumulated fatigue damage, bore loadings, and casing strain, and limiting said control signal on the basis of one of said limit signals.
 33. A method for operating a steam turbine comprising: a. determining steam temperature in a predetermined turbine region separate from and in heat transfer relation with a preselected turbine rotor portion; b. generating a representation of present heat flow from said region to said turbine rotor portion as a predetermined function of said determined steam temperature; and c. controlling the steam turbine speed as a function of said heat flow representation and continuously controlling the heat flow to said rotor substantially corresponding to maximum allowable rotor strain.
 34. The method as described in claim 33, wherein the step of controlling the steam turbine speed comprises: a. generating a difference signal representing the difference between a reference heat flow corresponding to maximum allowable rotor strain, and heat flow to the rotor; and b. generating a control signal as a function of said difference signal, and controlling said steam turbine speed by said control signal.
 35. The method as described in claim 34 further comprising generating limit signals representing limiting values of conditions selected from the group consisting of rotor surface strain, bore strain and casing strain, and limiting said control signal on the basis of one of said limit signals.
 36. A method for controlling a steam turbine comprising: a. generating a representation of current heat flow to the turbine rotor from a predetermined region in heat transfer relation to said rotor; and b. continuously controlling steam flow to said turbine as a function of said current heat flow representation.
 37. The method for control of a steam turbine as described in claim 36, comprising controlling said steam flow so as to change turbine speed.
 38. The method for control of a steam turbine as described in claim 36, comprising controlling said steam flow so as to change the load carried by said steam turbine.
 39. The method of controlling a steam turbine as described in claim 36, wherein said heat flow representation is generated as a function of the impulse chamber steam temperature (TI), the rotor surface temperature (TS), the heat transfer coefficient at the rotor surface (H), and the rotor surface area (A).
 40. The method of controlling a steam turbine as described in claim 39, wherein H is determined as a function of rotor speed and steam flow past the rotor.
 41. The method of controlling a steam turbine as described in claim 40, comprising determining at least one other turbine thermal condition, and limiting said steam flow to said turbine when said thermal condition exceeds a predetermined limit.
 42. The method of controlling a steam turbine as described in claim 41, wherein said thermal condition is rotor surface strain.
 43. The method of controlling a steam turbine as described in claim 41, wherein said thErmal condition is casing strain.
 44. A method for controlling steam turbine operation using a digital computer comprising: a. determining the temperature difference between the steam temperature in a predetermined turbine region in heat transfer relation with the turbine rotor surface and the temperature of such turbine rotor surface; b. determining the rotor speed and steam flow through said predetermined region; c. generating, with a general purpose programmed digital computer, a turbine operating representation as a function of said temperature difference, rotor speed and steam flow; and d. controlling steam flow to said turbine as a function of said operating representation.
 45. The digital computer control method as described in claim 44 wherein said operating representation is a function of heat flow to the rotor surface, and further comprising the step of generating a heat flow limit representation as a function of a predetermined turbine thermal condition and limiting said turbine steam flow in accordance with said heat flow limit.
 46. The digital computer control method as described in claim 44, comprising performing the following steps with said general purpose programmed digital computer: a. generating a representation of the heat transfer coefficient at said rotor surface; b. generating a representation of present heat flow to said rotor surface as a function of said heat transfer coefficient and said temperature difference; c. generating a representation of the heat flow difference between a reference heat flow corresponding to maximum heat flow for allowable rotor strain, and present heat flow; and d. generating said operating representation as a function of said difference representation.
 47. The digital computer control method as described in claim 46, wherein said operating representation is a speed representation.
 48. A digital computer control method as described in claim 46, wherein said operating representation is a load representation.
 49. An improved steam turbine system comprising: a. a steam turbine having a portion subject to thermal stress when said turbine is in operation; b. means for generating a first representation of a first predetermined temperature condition at said portion; c. means for generating a second representation of a second predetermined temperature condition at a predetermined temperature region in heat transfer relation with said portion; d. means for utilizing said first and second representations for generating a representation of heat flow to said portion as a function of said first and second temperature conditions; and e. means for continuously controlling operation of said steam turbine as a function of said heat flow representation, whereby said thermal stress is controlled.
 50. An improved steam turbine system comprising: a. a steam turbine having a portion subject to thermal stress when said turbine is in operation; b. means for generating a first representation of a first predetermined temperature condition at said portion; c. means for generating a second representation of a second predetermined temperature condition at a predetermined temperature region in heat transfer relation with said portion; d. means utilizing said first and second representations for generating a representation of the rate of change of said first predetermined temperature condition; and e. means for controlling operation of said steam turbine as a function of said rate of change representation, whereby said thermal stress is controlled. 